Suction structure in double-headed piston type compressor

ABSTRACT

A suction structure is provided for allowing refrigerant into first and second compression chambers from a suction pressure region through first and second rotary valves and first and second communication passages in a double-headed piston type compressor. The first and the second rotary valves respectively have first and second introduction passages. The distance to the first communication passage from the suction pressure region through the first introduction passage is greater than the distance to the second communication passage from the suction pressure region through the second introduction passage. The first communication passage with a circular cross-section in the cylinder block connects the first compression chamber to the first introduction passage. The second communication passage with a circular cross-section in the cylinder block connects the second compression chamber to the second introduction passage. The diameter of the first communication passage is greater than the diameter of the second communication passage.

BACKGROUND OF THE INVENTION

The present invention relates to a suction structure for allowing refrigerant from a suction pressure region into compression chambers in a double-headed piston type compressor. More specifically, the compressor has rotary valves rotated integrally with a rotary shaft and having introduction passages for introducing refrigerant from the suction pressure region into the compression chambers defined in cylinder bores by the double-headed pistons.

In double-headed piston type compressors, there are two types of suction valves. One is a rotary valve as disclosed in Unexamined Japanese Patent Publication No. 2007-032445. The other is a reed type suction valve as disclosed in Unexamined Japanese Patent Publication No. 2000-145629. The piston type compressor including the rotary valves has lower suction resistance in introducing refrigerant into cylinder bores, and has superior energy efficiency, compared to the piston type compressor including the reed type suction valves.

In the compressor disclosed in the above reference No. 2007-032445, pairs of a front cylinder bore and a rear cylinder bore accommodate double-headed pistons therein, and the pistons are reciprocated in accordance with the rotation of a rotary shaft. Each double-headed piston defines a front compression chamber in the front cylinder bore and a rear compression chamber in the rear cylinder bore. The rotary shaft has a front rotary valve and a rear rotary valve which are formed integrally therewith. A supply passage is formed in the rotary shaft, and outlets of the supply passage are formed in the front and the rear rotary valves. Communication passages are formed in cylinder blocks so as to communicate with the compression chambers. The outlets of the supply passage intermittently communicate with the communication passages in accordance with the rotation of the rotary shaft, or, the rotation of the rotary valves. When the outlets of the supply passage communicate with the communication passages, the refrigerant in the supply passage is introduced into the compression chambers.

Generally, the communication passages have elongated cross-sections, as disclosed in Unexamined Japanese Patent Publication No. 6-129350. The elongated holes are formed such that the dimensions of elongated holes in the axial direction of the rotary shaft are larger than the dimensions thereof in the circumferential direction. The elongated holes are applied in order to collect the residual gas in the compression chambers thereby improving the volume efficiency.

The supply passage communicates with the suction chamber formed in the rear housing so as to supply refrigerant in the suction chamber to the front and rear compression chambers therethrough. The refrigerant in the front compression chamber is discharged to the front discharge chamber formed in the front housing, by pushing open the respective discharge valve. The refrigerant in the rear compression chamber is discharged to the rear discharge chamber formed in the rear housing, by pushing open the respective discharge valve.

The pressure in the front discharge chamber is equal to the pressure in the rear discharge chamber. The refrigerant in the compression chambers is compressed up to the pressure level in the discharge chambers. Therefore, as the amount of the refrigerant introduced into the compression chambers is decreased, the compression rate is increased. When the compression rate is increased, the temperature of the compressed refrigerant rises, accordingly.

The distance to the front compression chamber from the suction chamber through the supply passage is greater than the distance to the rear compression chamber from the suction chamber through the supply passage. Therefore, the refrigerant amount into the front compression chamber is smaller than that into the rear compression chamber during the time when the communication passages communicate with the outlets of the rotary valves, respectively. The temperature of the refrigerant compressed in the front compression chamber becomes higher than the temperature of the refrigerant compressed in the rear compression chamber, accordingly.

When the temperature of the refrigerant compressed in the front compression chamber becomes excessively high, the temperature of the front housing rises. The sealing function may be deteriorated in seal members interposed between the front housing and the cylinder block, accordingly.

The present invention is directed to suppress increase in temperature of refrigerant compressed in compression chambers in a double-headed piston type compressor.

SUMMARY OF THE INVENTION

In accordance with the present invention, a suction structure is provided for allowing refrigerant from a suction pressure region in a double-headed piston type compressor. The compressor has a cylinder block, a rotary shaft, a double-headed piston, first and second cylinder bores, and first and second compression chambers. The rotary shaft is supported by the cylinder block. The double-headed piston reciprocates with rotation of the rotary shaft. The first and the second cylinder bores are formed in the cylinder block in a paired manner so as to accommodate the double-headed piston. The first and the second compression chambers are defined by the double-headed piston in the first and the second cylinder bores, respectively. The suction structure in the compressor includes first and second rotary valves, and first and second communication passages. The first rotary valve introduces refrigerant from a suction pressure region into the first compression chamber through a first introduction passage. The second rotary valve introduces refrigerant from the suction pressure region into the second compression chamber through a second introduction passage. Each part of the first and the second introduction passages is formed in the rotary shaft. The first communication passage has a circular cross-section and is formed in the cylinder block so as to connect the first compression chamber to the first introduction passage. The second communication passage has a circular cross-section and is formed in the cylinder block so as to connect the second compression chamber to the second introduction passage. The distance to the first communication passage from the suction pressure region through the first introduction passage is greater than the distance to the second communication passage from the suction pressure region through the second introduction passage. The diameter of the first communication passage is greater than the diameter of the second communication passage.

Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:

FIG. 1 is a longitudinal cross-sectional view of a compressor according to a preferred embodiment of the present invention;

FIG. 2A is a partially enlarged cross-sectional view of the compressor according to the preferred embodiment;

FIG. 2B is a cross-sectional view which is taken along the line III-III in FIG. 2A;

FIG. 2C is a cross-sectional view which is taken along the line IV-IV in FIG. 2A;

FIG. 2D is a graph showing relation between temperature in front discharge chamber and cross-section ratio with regard to area of first communication passage to that of second communication passage according to the preferred embodiment;

FIG. 3A is a cross-sectional view which is taken along the line I-I in FIG. 1; and

FIG. 3B is a cross-sectional view which is taken along the line II-II in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of a double-headed piston type compressor 10 according to the present invention will be described with reference to FIGS. 1 through 3. It is noted that the front side and the rear side of the double-headed piston type compressor 10 respectively correspond to the left side and the right side in the drawings. In addition, the front side and the rear side of the compressor 10 respectively serve as a first side and a second side. Referring to FIG. 1, a front cylinder block 11 is joined to a rear cylinder block 12. A front housing 13 is joined to the front cylinder block 11. A rear housing 14 is joined to the rear cylinder block 12. The front and rear cylinder blocks 11, 12 and the front and rear housings 13, 14 constitute a whole compressor housing assembly of the double-headed piston type compressor 10. A front discharge chamber 131 as a discharge pressure region in the compressor 10 is defined in the front housing 13. A rear discharge chamber 141 as a discharge pressure region in the compressor 10 is defined in the rear housing 14. A suction chamber 142 as a suction pressure region is defined in the rear housing 14. It is noted that “in the compressor” corresponds to the inside of the whole compressor housing assembly of the compressor 10, and that “out of the compressor” corresponds to the outside of the whole compressor housing assembly.

A valve port plate 15, a valve plate 16, and a retainer plate 17 are interposed between the front cylinder block 11 and the front housing 13. A valve port plate 18, a valve plate 19, and a retainer plate 20 are interposed between the rear cylinder block 12 and the rear housing 14. Discharge ports 151, 181 are respectively formed in the valve port plates 15, 18. Discharge valves 161, 191 are respectively formed in the valve plates 16, 19 to open and close the respective discharge ports 151, 181. Retainers 171, 201 are respectively formed in the retainer plates 17, 20 to regulate the respective opening degrees of the discharge valves 161, 191.

Gaskets which are not shown in the drawings are respectively interposed between the front and rear cylinder blocks 11, 12, between the front cylinder block 11 and the front housing 13, and between the rear cylinder block 12 and the rear housing 14. The gaskets are made of metal plates whose both side surfaces are covered by rubber sealing layers. The gaskets serve to prevent leakage of refrigerant gas through the clearances between the cylinder blocks 11, 12, between the cylinder block 11 and the front housing 13, and between the cylinder block 12 and the rear housing 14.

A rotary shaft 21 is rotatably supported by the front and rear cylinder blocks 11, 12 and is inserted into shaft holes 111, 121 which extend through the front and rear cylinder blocks 11, 12. The outer periphery of the rotary shaft 21 is in contact with the inner peripheries of the shaft holes 111, 121. The rotary shaft 21 is directly supported by the front and rear cylinder blocks 11, 12 through the inner peripheries of the respective shaft holes 111, 121. A contacting portion of the outer periphery of the rotary shaft 21 with the shaft hole 111 forms a sealing circumferential surface 211. A contacting portion of the outer periphery of the rotary shaft 21 with the shaft hole 121 forms a sealing circumferential surface 212.

A swash plate 23 as a cam body is secured to the rotary shaft 21. The swash plate 23 is accommodated in a crank chamber 24 which is defined between the front and rear cylinder blocks 11, 12. A lip-seal type shaft seal member 22 is interposed between the front housing 13 and the rotary shaft 21. The shaft seal member 22 prevents leakage of the refrigerant gas through the clearance between the front housing 13 and the rotary shaft 21. The front end of the rotary shaft 21 protruding externally from the front housing 13 is connected to a vehicle engine 26 as an external drive source through an electromagnetic clutch 25. The rotary shaft 21 receives driving force for rotation from the vehicle engine 26 through the electromagnetic clutch 25.

As shown in FIG. 3A, a plurality of first cylinder bores 27 is formed in the front cylinder block 11, and is arranged around the rotary shaft 21. As shown in FIG. 3B, a plurality of second cylinder bores 28 is formed in the rear cylinder block 12, and is arranged around the rotary shaft 21. A double-headed piston 29 is accommodated in each pair of the cylinder bores 27, 28.

As shown in FIG. 1, the double-headed pistons 29 are engaged with the swash plate 23 through a pair of shoes 30. The swash plate 23 integrally rotates with the rotary shaft 21. The rotary motion of the swash plate 23 is transmitted to the double-headed pistons 29 through the shoes 30 so that each double-headed piston 29 reciprocates in the respective pair of the cylinder bores 27, 28. Each double-headed piston 29 has a first cylindrical head 291 which defines a first compression chamber 271 in the respective first cylinder bore 27. Each double-headed piston 29 has a second cylindrical head 292 at the opposite end to the first head 291, and the second head 292 defines a second compression chamber 281 in the respective second cylinder bore 28.

An in-shaft passage 31 is formed in the rotary shaft 21. The in-shaft passage 31 extends along the rotary axis 210 of the rotary shaft 21. An inlet 311 of the in-shaft passage 31 is open to the suction chamber 142 in the rear housing 14. A first outlet 312 of the in-shaft passage 31 is open at the front sealing circumferential surface 211 of the rotary shaft 21 in the shaft hole 111. A second outlet 313 of the in-shaft passage 31 is open at the rear sealing circumferential surface 212 of the rotary shaft 21 in the shaft hole 121.

As shown in FIGS. 2A and 3A, a first communication passage 32 is formed in the front cylinder block 11 so as to communicate with the first cylinder bores 27 and the shaft hole 111. As shown in FIGS. 2B and 3B, a second communication passage 33 is formed in the rear cylinder block 12 so as to communicate with the second cylinder bores 28 and the shaft hole 121. As the rotary shaft 21 rotates, the first and second outlets 312, 313 of the in-shaft passage 31 intermittently communicate with the first and second communication passages 32, 33, respectively.

When one of the first cylinder bores 27 is in a suction process, that is, in a process of the double-headed piston 29 moving from the left side to the right side in FIG. 1, the first outlet 312 communicates with the first communication passage 32. As a result, refrigerant in the suction chamber 142 is introduced into the first compression chamber 271 in the first cylinder bore 27 through the in-shaft passage 31, the first outlet 312, and the first communication passage 32.

When the first cylinder bore 27 is in a discharge process, that is, in a process of the double-headed piston 29 moving from the right side to the left side in FIG. 1, the communication between the first outlet 312 and the first communication passage 32 is shut off. As a result, refrigerant in the first compression chamber 271 is discharged to the front discharge chamber 131 through the discharge port 151 by pushing open the discharge valve 161. The refrigerant discharged to the discharge chamber 131 flows out to an external refrigerant circuit 34 through a passage 341.

When one of the second cylinder bores 28 is in a suction process, that is, in a process of the double-headed piston 29 moving from the right side to the left side in FIG. 1, the second outlet 313 communicates with the second communication passage 33. As a result, refrigerant in the suction chamber 142 is introduced into the second compression chamber 281 of the second cylinder bore 28 through the in-shaft passage 31, the second outlet 313, and the second communication passage 33.

When the second cylinder bore 28 is in a discharge process, that is, in a process of the double-headed piston 29 moving from the left side to the right side in FIG. 1, the communication between the second outlet 313 and the second communication passage 33 is shut off. As a result, refrigerant in the second compression chamber 281 is discharged to the rear discharge chamber 141 through the discharge port 181 by pushing open the discharge valve 191. The refrigerant discharged to the discharge chamber 141 flows out to the external refrigerant circuit 34 through a passage 342.

The external refrigerant circuit 34 is provided with a heat exchanger 37 for removing heat from refrigerant, an expansion valve 38, and a heat exchanger 39 for evaporating the refrigerant with heat. The expansion valve 38 controls the flow rate of the refrigerant in accordance with the fluctuation in temperature of the refrigerant gas at the outlet of the heat exchanger 39. The refrigerant flowing out to the external refrigerant circuit 34 returns to the suction chamber 142.

The part of the rotary shaft 21 corresponding to the sealing circumferential surface 211 forms a first rotary valve 35. The part of the rotary shaft 21 corresponding to the sealing circumferential surface 212 forms a second rotary valve 36. The rotary valves 35, 36 are formed integrally with the rotary shaft 21. The in-shaft passage 31 and the first outlet 312 form a first introduction passage 40 for the rotary valve 35. The in-shaft passage 31 and the second outlet 313 form a second introduction passage 41 for the rotary valves 36. Part of the second introduction passage 41 in the rotary shaft 21 shares part of the first introduction passage 40. The length of the first introduction passage 40 is greater than the length of the second introduction passage 41. That is, the distance to the first communication passage 32 from the suction chamber 142 through the first introduction passage 40 is greater than the distance to the second communication passage 33 from the suction chamber 142 through the second introduction passage 41.

As shown in FIG. 1, the activation of the electromagnetic clutch 25 is controlled by a computer C. The computer C is connected to a switch W for operating an air conditioner, a setting device S for setting a target room temperature, and a detecting device F for detecting a room temperature. When the switch W is turned on, the computer C controls the electric current for activation and deactivation to the electromagnetic clutch 25 in accordance with the temperature difference between the target room temperature and the detected room temperature.

The computer C turns off the electric current to the electromagnetic clutch 25, when the detected temperature is lower than the target temperature, or, when the detected temperature is higher than the target temperature and the temperature difference is within an allowable range. In this case, the electromagnetic clutch 25 is in a disengaged state, and the driving force of the vehicle engine 26 is not transmitted to the rotary shaft 21. The computer C supplies electric current to the electromagnetic clutch 25, when the detected temperature is higher than the target temperature and the temperature difference between the detected temperature and the target temperature is beyond the allowable level. In this case, the electromagnetic clutch 25 is in an engaged state, and the driving force of the vehicle engine 26 is transmitted to the rotary shaft 21.

As shown in FIG. 2B, the first communication passage 32 has a circular cross-section. As shown in FIG. 2C, the second communication passage 33 has a circular cross-section. The diameter D of the first communication passage 32 is set larger than the diameter d of the second communication passage 33. The in-shaft passage 31 has a circular cross-section, and the diameter of the in-shaft passage 31 is set larger than the diameter D of the first communication passage 32.

In FIG. 2D, a curve E indicates change in temperature of the front discharge chamber 131 under such a condition that the diameter D of the first communication passage 32 is varied while the diameter d of the second communication passage 33 is held constant. Points on the curve E indicate actually measured values. The horizontal axis indicates a cross section ratio of a cross-sectional area of first communication passage to a cross-sectional area of second communication passage. A cross-sectional ratio of this embodiment is expressed by (πD²/4)/(πd²/4), whose (πD²/4) means a cross-sectional area of the first communication passage 32, and (πd²/4) means a cross-sectional area of the second communication passage 33. The vertical axis indicates temperature in the front discharge chamber 131.

A curve G in the graph of FIG. 2D is under a condition having communication passages with respective elongated cross-sections (for example, as shown in the reference No. 6-129350 as a background art). The dimension of the respective elongated cross-section in the axial direction of the rotary shaft 21 is larger than the dimension in the circumferential direction. The curve G indicates change in temperature in the discharge chamber 131 against the cross section ratio as the elongated cross-sectional area is varied.

In the both curves E and G, the total volumes of the first and second compression chambers 271, 281 are 200 cc, and the rotational speed of the double-headed piston type compressor 10 is 4500 rpm, and the ratio of the discharge pressure Pd to the suction pressure Ps is 12.

As shown in the graph of FIG. 2D, the temperature in the discharge chamber 131 indicated by the curve E is lower than the lowest temperature indicated by the curve G in the range where the cross section ratio is less than or equal to 1.8. That is the case in that the diameter D of the first communication passage 32 is larger than the diameter d of the second communication passage 33, and additionally less than or equal to 1.8 times of the diameter d. Especially, the temperature at the cross section ratio of 1.4 becomes equal to the temperature at the cross section ratio of 1. That means, if the diameter D of the first communication passage 32 is larger than the diameter d of the second communication passage 33 and less than or equal to 1.4 times of the diameter d, the temperature in the discharge chamber 131 becomes lower than that in case where the diameters D, d equal. When the diameter D of the first communication passage 32 is 1.2 times of the diameter d of the second communication passage 33, the temperature in the discharge chamber 131 is the lowest.

According to the preferred embodiment, the following advantageous effects are obtained.

(1) The distance to the first compression chamber 271 from the suction chamber 142 through the first introduction passage 40 is greater than the distance to the second compression chamber 281 from the suction chamber 142 through the second introduction passage 41. In this case, the refrigerant amount into the first compression chamber 271 through the first communication passage 32 may be smaller than that into the second compression chamber 28i through the second communication passage 33, supposing that the cross-sectional areas of the communication passages 32, 33 equal. The compression ratio in the first compression chamber 271 may become higher than the compression ratio in the second compression chamber 281, accordingly. Thereby the temperature in the front discharge chamber 131 may become higher than the temperature in the rear discharge chamber 141.

In this embodiment, the diameter D of the first communication passage 32 is set larger than the diameter d of the second communication passage 33 so that the cross-sectional area of the first communication passage 32 is set larger than that of the second communication passage 33. Such a structure reduces the difference between the refrigerant amount into the first compression chamber 271 through the first communication passage 32 and the refrigerant amount into the second compression chamber 281 through the second communication passage 33. By decreasing the difference in refrigerant amount, the temperature increase of the refrigerant compressed in the first compression chamber 271 is effectively suppressed.

(2) The first and second communication passages 32, 33 with the circular cross-sections are easily manufactured.

(3) When the diameter D of the first communication passage 32 is larger than the diameter d of the second communication passage 33 and less than or equal to 1.8 times of the diameter d, the temperature increase in the refrigerant compressed in the first compression chamber 271 is effectively suppressed.

(4) When the diameter D is larger than the diameter d and less than or equal to 1.4 times of the diameter d, the temperature of the refrigerant compressed in the first compression chamber 271 is effectively decreased, compared to a case where the diameters D, d equal.

(5) When the diameter D is 1.2 times larger than the diameter d, the temperature increase of the refrigerant compressed in the first compression chamber 271 is further effectively suppressed.

(6) Communication passages as a comparative conventional art have elongated cross-sections. The dimension of each elongated cross-section in the axial direction is set larger than the dimension in the circumferential direction. Supposing the cross-sectional areas of the comparative communication passages are equal to those of the communication passages 32, 33, the diameters of the communication passages 32, 33 are larger than the widths of the elongated cross-sections of the comparative communication passages (in the circumferential direction of the rotary valves 35, 36). Therefore, in accordance with a rotation of the rotary valves 35, 36, the timing of opening the communication passages 32, 33 is earlier than the timing of opening the elongated cross-sectional communication passages. It is noted that “the time of opening” represents the time when the outlets 312, 313 of the in-shaft passage 31 start to communicate with the communication passages 32, 33. Similarly, in accordance with a rotation of the rotary valves 35, 36, the timing of closing the communication passages 32, 33 is later than the timing of closing the elongated cross-sectional communication passages. It is also noted that “the timing of closing” represents the time when the state of the outlets 312, 313 of the in-shaft passage 31 is changed from the connecting state to the disconnecting state. Therefore, with the structure having the circular cross-sectional communication passages 32, 33, the supply of the refrigerant into the compression chambers 271, 281 is appropriately increased, compared to the elongated cross-sectional communication passages.

(7) As dimensions Z1, Z2 (as shown in FIG. 1) of the communication passages 32, 33 measured in the axial direction are set longer, lengths X, Y (as shown in FIG. 1) of the heads 291, 292 of the double-headed pistons 29 in the axial direction are required to be set longer. As the lengths X, Y of the heads 291, 292 are set longer, the weight of the double-headed pistons 29 may increase.

The diameters of the circular cross-sectional communication passages 32, 33 are smaller than the dimensions in the axial direction of the elongated cross-sections of the comparative communication passages, which respectively have the same cross-sectional areas as the communication passages 32, 33. Therefore, with the structure having the circular cross-sectional communication passages 32, 33, the lengths X, Y of the heads 291, 292 are appropriately shortened, compared to the communication passages having the elongated cross-sections.

(8) The circular cross-sections of the communication passages 32, 33 shorten the dimensions Z1, Z2 of the communication passages 32, 33 in the axial direction, compared to the case having the elongated cross-sectional communication passages. In the suction process, a timing when the end faces of the double-headed pistons 29 complete to pass by the communication passages 32, 33 becomes earlier than the case having the elongated cross-sectional communication passages, accordingly. That is, the timing when the double-headed pistons 29 fully open the communication passages 32, 33 is set earlier than the case with the elongated cross-sectional communication passages, thereby increasing the supply of the amount of the refrigerant.

The present invention is not limited to the above-described embodiment, but may be modified into the following alternative embodiments.

The suction chamber may be formed in the front housing 13 so as to introduce the refrigerant into the compression chambers 271, 281 through the in-shaft passage 31.

The suction pressure region may be provided outside of the compressor so as to introduce the refrigerant into the first and second introduction passages. The first and second rotary valves 35, 36 may be formed independently of the rotary shaft 21.

Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein but may be modified within the scope of the appended claims. 

1. A suction structure for allowing refrigerant from a suction pressure region in a double-headed piston type compressor, the compressor comprising: a cylinder block; a rotary shaft supported by the cylinder block; a double-headed piston reciprocating with rotation of the rotary shaft; a first cylinder bore and a second cylinder bore formed in the cylinder block in a paired manner so as to accommodate the double-headed piston; a first compression chamber and a second compression chamber defined by the double-headed piston in the first and the second cylinder bores, respectively; the suction structure in the compressor comprising: a first rotary valve introducing refrigerant from the suction pressure region into the first compression chamber through a first introduction passage; a second rotary valve introducing refrigerant from the suction pressure region into the second compression chamber through a second introduction passage, wherein each part of the first and the second introduction passages is formed in the rotary shaft; a first communication passage having a circular cross-section and formed in the cylinder block so as to connect the first compression chamber to the first introduction passage; and a second communication passage having a circular cross-section and formed in the cylinder block so as to connect the second compression chamber to the second introduction passage, wherein the distance to the first communication passage from the suction pressure region through the first introduction passage is greater than the distance to the second communication passage from the suction pressure region through the second introduction passage, wherein the diameter of the first communication passage is greater than the diameter of the second communication passage.
 2. The suction structure according to claim 1, wherein the diameter of the first communication passage is less than or equal to 1.8 times of the diameter of the second communication passage.
 3. The suction structure according to claim 2, wherein the diameter of the first communication passage is less than or equal to 1.4 times of the diameter of the second communication passage.
 4. The suction structure according to claim 1, wherein the second introduction passage in the rotary shaft shares a part of the first introduction passage.
 5. A double-headed piston type compressor comprising: a cylinder block; a rotary shaft supported by the cylinder block; a cam body formed with the rotary shaft; a double-headed piston engaged with the cam body, wherein rotation of the rotary shaft is transmitted to the piston through the cam body; a first cylinder bore and a second cylinder bore formed in the cylinder block in a paired manner so as to accommodate the double-headed piston; a first compression chamber and a second compression chamber defined by the double-headed piston in the first and the second cylinder bores, respectively; a first rotary valve rotated integrally with the rotary shaft, wherein the rotary valve has a first introduction passage so as to introduce refrigerant from a suction pressure region into the first compression chamber through the first introduction passage; a second rotary valve rotated integrally with the rotary shaft, wherein the second rotary valve has a second introduction passage so as to introduce refrigerant from the suction pressure region into the second compression chamber through the second introduction passage, wherein each part of the first and second introduction passages is formed in the rotary shaft; a first communication passage having a circular cross-section and formed in the cylinder block so as to connect the first compression chamber to the first introduction passage; and a second communication passage having a circular cross-section and formed in the cylinder block so as to connect the second compression chamber to the second introduction passage, wherein the distance to the first communication passage from the suction pressure region through the first introduction passage is greater than the distance to the second communication passage from the suction pressure region through the second introduction passage, wherein the diameter of the first communication passage is greater than the diameter of the second communication passage. 